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Electrochemistry of coordination compounds
J. Electroanal. Chem., 63 (1975)421-423
421
© ElsevierSequoia S.A., Lausanne Printed in The Netherlands
S H O R T COMMUNICATION
Electrochemistry of coordination compounds VII. Comparison between the polarographic behaviour and the nucleophilic ~bstitution reactions in platinum(II) complexes MARIO MARTELLI Consiglio Nazionale delle Ricerche, Centro di Studio sulla Stabilitd e Reattivitdl dei Composti di Coordinazione, c/o Istituto di Chimica Analitica dell'Universitd, Via Loredan 4, 35100 Padua (Italy)
GIANNI ZOTTI and GIUSEPPE PILLONI Consiglio Nazionale delle Ricerche, Laboratorio di Polarografia ed Elettrochimica Preparativa, Via Monte Cengio 33, 35100 Padua (Italy)
(Received5th March 1975)
Among the d 8 systems, the square planar complexes of Pt(II) have been the subject of by far the most extensive kinetic investigations ~-3. However the extrakinetic data which contribute towards understanding of the changes in the activation energy or in the rate of substitution of the coordinated ligand are few 1. In this field, electrochemical studies would provide particularly useful data. In fact, previously we found a correlation between the electron uptake abilities of square planar complexes of the Ni triad and the free energy changes going from the four coordinate ground state to the five coordinate transition state in the SN2 reactions at the same systems4. Now, we extended the investigation to trans-[Pt(PEt3)2CI2] (A), trans-[Pt(piperidine)2C12] (C), trans-[Pt(piperidine)(PEt3)C12] (B) tor which detailed kinetic behaviour has been reported 5. Experimental Trans-[Pt(piperidine)(PEt3)Cl2] has been prepared with a procedure similar
to that described by Chatt and Venanzi 6 by reaction of Pt2CI4" 2 PEt3 (76 mg) with piperidine (0.2 ml) in acetone at room temperature m.p. 72°C. Elemental analysis: found Pt 41.3%, N 2.89%, C 28.11%; calculated Pt 41.6%, N 2.98%, C 28.15%. Trans-[Pt(PEt3)2C12 ] and trans-[Pt(piperidine)2C12] were prepared as reported in the literature 7'8. The solvent, 1,2-dimethoxyethane, was purified by refluxing over LiA1H4. It was. stored over LiA1H# until needed and then distilled under nitrogen. All solutions contained 0.1 M tetrabutylammonium perchlorate, TBAP, as supporting electrolyte. The preparation of TBAP, the electrochemical equipment and the methodology employed have been described previously4.
~22
SHORT COMMUNICATION
Results and discussion
The electrochemical reduction at the dropping mercury electrode of the three complexes occurs in a single two-electron irreversible and diffusion controlled step. The half-wave potentials are: -1.87 (A), -1.96 (B), -2.10 (C) V vs. SCE. Since the reduction pathway is for all the complexes almost certainly identical, as supported by the identical number of electrons accepted and by the identical structure of the complexes, the half-wave potentials are comparable and are a measure of the energy which the depolarizer needs to give an electronic rearrangement able to facilitate the electron transfer 9. This energy is in the order A < B < C meaning that the phosphine complex takes up electrons more easily from the electrode in comparison with the piperidine and the "mixed" complexes. On the basis of these results it is reasonable to expect that in the quite analogous assumptions of electrons such as in the SN2 reactions 5, the phosphine-containing complex shows a larger contribution of the nucleophiles to the formation state, i.e. the rate of reaction is very sensitive to changes in the entering group. In fact, taking into consideration the SN2 reactions we note that between the ease of accepting electrons from the electrode, A > B > C, and the nucleophilic discrimination factors, that is 1.62 (A), 1.44 (B), 0.95 (C), exists a linear correlation 3' ~. This seems to indicate that the nucleophilic discrimination factor is a measure of the ease of accepting electrons at least as far as the complexes examined are concerned. The parallelism between the kinetic and polarographic behaviour is even stronger if we observe that the nucleophilic discrimination factor as well as the ease of accepting electrons of B are intermediate between those of A and C. Once more it appears, in close analogy with our previous interpretation of the kinetic results ~, that each ligand contributes with its specific ability to promote the assumption of electrons. Finally all these parallelisms between electrochemical and nucleophilic assumption of electrons allow us to make some hypotheses on the reduction mechanism of the d 8 systems at the electrode. At least two possible pathways are available namely via bridging between the chloride ligand and the electrode or via the vacant Pz orbital2- For two reasons we favour the path offered by the vacant Pz orbital: (i) this orbital is believed to accommodate the two electrons donated by an entering ligand in SN2 reactions 2, (ii) the high negative reduction potentials make the bridging hypothesis quite unlikely 1°'11 while a contact between the reacting particle and the electrode can be effected through the positively charged central metal atom. Acknowledoment
The authors wish to thank Prof. A. Turco for valuable and stimulating discussion. REFERENCES 1 2 3 4 5
F. Basolo and R, G. Pearson, Mechanisms of Inorganic Reactions, Wiley, New York, 1967, Ch. 5. C. H. Langford and H. B. Gray, Ligand Substitution Processes, Benjamin, New York, 1965, Ch. 2. L. Cattalini, Progress in Inorganic Chemistry, Vol. 13,.Interscience, New York, 1970, p. 263. M. Martelli, G. Pilloni, G. Zotti and S. Daolio, Inorg. Chim. Acta, 11 (1974) 155. M. Martelli and A. Orio, Ric. Sci., 35 (II A) (1965) 1089.
SHORT COMMUNICATION 6 7 8 9 10 11
J. Chatt and L. M. Venanzi, J. Chem~ Soc., (1955) 3858. K. A. Jensen, Z. Anorg. Chem., 229 (1936) 225. J. Chatt, L. A. Duncanson and L. M. Venanzi, J. Chem. Soe., (1955) 4461. A. A. Vlcek, Progress in Polarooraphy, Vol. 1, Interscience, New York, 1962, p. 281. P. Kivalo, Suomen Kemistilehti, B 30 (1957) 208. A. M. Frumkin and N. V. Nikolaeva, J. Chem. Phys., 26 (1957) 1552.
423
421
© ElsevierSequoia S.A., Lausanne Printed in The Netherlands
S H O R T COMMUNICATION
Electrochemistry of coordination compounds VII. Comparison between the polarographic behaviour and the nucleophilic ~bstitution reactions in platinum(II) complexes MARIO MARTELLI Consiglio Nazionale delle Ricerche, Centro di Studio sulla Stabilitd e Reattivitdl dei Composti di Coordinazione, c/o Istituto di Chimica Analitica dell'Universitd, Via Loredan 4, 35100 Padua (Italy)
GIANNI ZOTTI and GIUSEPPE PILLONI Consiglio Nazionale delle Ricerche, Laboratorio di Polarografia ed Elettrochimica Preparativa, Via Monte Cengio 33, 35100 Padua (Italy)
(Received5th March 1975)
Among the d 8 systems, the square planar complexes of Pt(II) have been the subject of by far the most extensive kinetic investigations ~-3. However the extrakinetic data which contribute towards understanding of the changes in the activation energy or in the rate of substitution of the coordinated ligand are few 1. In this field, electrochemical studies would provide particularly useful data. In fact, previously we found a correlation between the electron uptake abilities of square planar complexes of the Ni triad and the free energy changes going from the four coordinate ground state to the five coordinate transition state in the SN2 reactions at the same systems4. Now, we extended the investigation to trans-[Pt(PEt3)2CI2] (A), trans-[Pt(piperidine)2C12] (C), trans-[Pt(piperidine)(PEt3)C12] (B) tor which detailed kinetic behaviour has been reported 5. Experimental Trans-[Pt(piperidine)(PEt3)Cl2] has been prepared with a procedure similar
to that described by Chatt and Venanzi 6 by reaction of Pt2CI4" 2 PEt3 (76 mg) with piperidine (0.2 ml) in acetone at room temperature m.p. 72°C. Elemental analysis: found Pt 41.3%, N 2.89%, C 28.11%; calculated Pt 41.6%, N 2.98%, C 28.15%. Trans-[Pt(PEt3)2C12 ] and trans-[Pt(piperidine)2C12] were prepared as reported in the literature 7'8. The solvent, 1,2-dimethoxyethane, was purified by refluxing over LiA1H4. It was. stored over LiA1H# until needed and then distilled under nitrogen. All solutions contained 0.1 M tetrabutylammonium perchlorate, TBAP, as supporting electrolyte. The preparation of TBAP, the electrochemical equipment and the methodology employed have been described previously4.
~22
SHORT COMMUNICATION
Results and discussion
The electrochemical reduction at the dropping mercury electrode of the three complexes occurs in a single two-electron irreversible and diffusion controlled step. The half-wave potentials are: -1.87 (A), -1.96 (B), -2.10 (C) V vs. SCE. Since the reduction pathway is for all the complexes almost certainly identical, as supported by the identical number of electrons accepted and by the identical structure of the complexes, the half-wave potentials are comparable and are a measure of the energy which the depolarizer needs to give an electronic rearrangement able to facilitate the electron transfer 9. This energy is in the order A < B < C meaning that the phosphine complex takes up electrons more easily from the electrode in comparison with the piperidine and the "mixed" complexes. On the basis of these results it is reasonable to expect that in the quite analogous assumptions of electrons such as in the SN2 reactions 5, the phosphine-containing complex shows a larger contribution of the nucleophiles to the formation state, i.e. the rate of reaction is very sensitive to changes in the entering group. In fact, taking into consideration the SN2 reactions we note that between the ease of accepting electrons from the electrode, A > B > C, and the nucleophilic discrimination factors, that is 1.62 (A), 1.44 (B), 0.95 (C), exists a linear correlation 3' ~. This seems to indicate that the nucleophilic discrimination factor is a measure of the ease of accepting electrons at least as far as the complexes examined are concerned. The parallelism between the kinetic and polarographic behaviour is even stronger if we observe that the nucleophilic discrimination factor as well as the ease of accepting electrons of B are intermediate between those of A and C. Once more it appears, in close analogy with our previous interpretation of the kinetic results ~, that each ligand contributes with its specific ability to promote the assumption of electrons. Finally all these parallelisms between electrochemical and nucleophilic assumption of electrons allow us to make some hypotheses on the reduction mechanism of the d 8 systems at the electrode. At least two possible pathways are available namely via bridging between the chloride ligand and the electrode or via the vacant Pz orbital2- For two reasons we favour the path offered by the vacant Pz orbital: (i) this orbital is believed to accommodate the two electrons donated by an entering ligand in SN2 reactions 2, (ii) the high negative reduction potentials make the bridging hypothesis quite unlikely 1°'11 while a contact between the reacting particle and the electrode can be effected through the positively charged central metal atom. Acknowledoment
The authors wish to thank Prof. A. Turco for valuable and stimulating discussion. REFERENCES 1 2 3 4 5
F. Basolo and R, G. Pearson, Mechanisms of Inorganic Reactions, Wiley, New York, 1967, Ch. 5. C. H. Langford and H. B. Gray, Ligand Substitution Processes, Benjamin, New York, 1965, Ch. 2. L. Cattalini, Progress in Inorganic Chemistry, Vol. 13,.Interscience, New York, 1970, p. 263. M. Martelli, G. Pilloni, G. Zotti and S. Daolio, Inorg. Chim. Acta, 11 (1974) 155. M. Martelli and A. Orio, Ric. Sci., 35 (II A) (1965) 1089.
SHORT COMMUNICATION 6 7 8 9 10 11
J. Chatt and L. M. Venanzi, J. Chem~ Soc., (1955) 3858. K. A. Jensen, Z. Anorg. Chem., 229 (1936) 225. J. Chatt, L. A. Duncanson and L. M. Venanzi, J. Chem. Soe., (1955) 4461. A. A. Vlcek, Progress in Polarooraphy, Vol. 1, Interscience, New York, 1962, p. 281. P. Kivalo, Suomen Kemistilehti, B 30 (1957) 208. A. M. Frumkin and N. V. Nikolaeva, J. Chem. Phys., 26 (1957) 1552.
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